Control device for diesel engine

ABSTRACT

A control device for a diesel engine has a valve timing switching mechanism capable of selectively switching between first opening/closing timing and second opening/closing timing for an exhaust valve. The first opening/closing timing is set such that the period for which the exhaust valve is open overlaps the period for which an intake valve is open, and that opening start timing for the exhaust valve is after bottom dead center of expansion stroke of a piston. The second opening/closing timing is set such that opening start timing for the exhaust valve is advanced compared with the opening start timing according to the first opening/closing timing. A control unit controls the valve timing switching mechanism so that the exhaust valve is opened and closed according to the first opening/closing timing, at least when the diesel engine is operating in a predetermined cold condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a control device for a diesel engine, specifically, a control device for a diesel engine equipped with a valve timing switching mechanism capable of changing opening/closing timing for an exhaust valve of the engine, and an exhaust after-treatment device having a function of purifying exhaust gas of the engine.

2. Description of the Related Art

A NOx catalyst for reducing NOx (nitrogen oxides) in exhaust gas of a diesel engine to purify the exhaust gas, a particulate filter for trapping particulate in exhaust gas to purify the exhaust gas, etc. have been conventionally used as exhaust after-treatment devices.

In recent years, for example an ammonia selective reduction type NOx catalyst (hereinafter referred to as SCR catalyst) is used as a NOx catalyst. In an exhaust after-treatment device including an SCR catalyst, urea-water is supplied to upstream of the SCR catalyst, and ammonia produced as a result of the hydrolyzation of urea-water caused by heat from exhaust gas is supplied to the SCR catalyst. The SRC catalyst adsorbs the ammonia supplied to the SCR catalyst, and the SCR catalyst promotes denitrifying reaction between ammonia and NOx in the exhaust gas. The NOx is reduced in this manner.

The SCR catalyst does not satisfactorily perform such exhaust purifying function until the temperature of exhaust gas flowing into the SCR catalyst rises to at least a level at which the SCR catalyst is activated. However, when the engine is cold, exhaust gas discharged from the engine is at low temperature, and an exhaust pipe, a pre-stage oxidizing catalyst, a particulate filter, etc. placed upstream of the SCR catalyst have not warmed enough. Thus, heat transfers from exhaust gas to these members, so that the exhaust flowing into the SCR catalyst has a greatly decreased temperature. Such decrease in exhaust temperature may prevent the SCR catalyst from performing the above-mentioned exhaust purifying function satisfactorily.

Problems caused by decrease in exhaust temperature are observed not only in SCR catalyst. Similar problems are observed in a variety of catalysts and a particulate filter used as exhaust after-treatment devices. Specifically, in a catalyst used as exhaust after-treatment device, decreased exhaust temperature does not allow activation of the catalyst, therefore does not allow the catalyst to perform the catalytic function satisfactorily. In a particulate filter, decreased exhaust temperature makes continuous regeneration difficult and allows accumulation of particulate in the particulate filter, which results in decline of particulate trapping function.

In order to solve the problems caused by decrease in exhaust temperature as mentioned above, Unexamined Japanese Patent Publication Hei 10-68332 (hereinafter referred to as Document 1) has proposed a valve timing control device designed to raise exhaust temperature by changing opening/closing timing for exhaust valves of the engine, when the engine is cold.

The control device of Document 1 uses a mechanism which can vary the opening/closing timing for exhaust valves by changing the phase of an exhaust cam shaft relative to a crank shaft. When the engine is at low temperature, the opening/closing timing for the exhaust valves is advanced by a predetermined crank angle. This intends to encourage the discharge of combustion heat from cylinders to the exhaust device side, thereby causing a catalyst and the like to be activated early by the combustion heat.

In the control device of Document 1, however, by changing the phase of the exhaust cam shaft relative to the crank shaft, the opening/closing timing for each exhaust valve is varied, while the period of time for which each exhaust valve is open remains unchanged. Thus, as a result of advancing the timing of start of opening of the exhaust valve, the timing of completion of closing of the exhaust valve is advanced before top dead center of exhaust stroke, so that the exhaust valve closes before the associated intake valve is opened. Consequently, when the exhaust valve closes, gases remaining in the cylinder is compressed again, and when the intake valve is opened after that, the remaining gases thus compressed are discharged through an intake port. This hinders fresh air from being sucked into the cylinder through the intake port, thereby causing a decrease in air excess ratio. This leads to problems such as production of a large amount of black smoke, and emission of HC (hydrocarbon) into the atmosphere.

Further, the control device of Document 1 changes the phase of the exhaust cam shaft relative to the crank shaft by supplying oil pressure. When the engine is cold, a working oil supplied is also at low temperature and therefore high in viscosity. Thus, a phase change mechanism cannot be controlled stably until the working oil warms to some extent and decreases in viscosity. Thus, it is conceivable to configure the control device of Document 1 to maintain the exhaust valve opening/closing timing at the most advanced setting, while the working oil is at low temperature. In this case, however, if the engine comes to operate under a high load before the working oil temperature rises enough, the exhaust valve opening/closing timing is maintained at the most advanced setting for a while, since the working oil cannot be discharged smoothly. Consequently, the above-mentioned reduction in the fresh air sucked into the cylinder continues. Although the reduction in fresh air, to which combustion heat is transferred, results in a rise in exhaust temperature, great reduction in fresh air may lead to a large amount of black smoke produced by incomplete combustion and an excessive rise in exhaust temperature.

SUMMARY OF THE INVENTION

An aspect of the present invention is directed to a control device for a diesel engine, comprising: a valve timing switching mechanism capable of selectively switching between first opening/closing timing and second opening/closing timing for an exhaust valve of the diesel engine; an exhaust after-treatment device for purifying exhaust gas discharged from the diesel engine; and a control means for controlling the valve timing switching mechanism so that the exhaust valve is opened and closed according to the first opening/closing timing, at least when the diesel engine is operating in a predetermined cold condition, wherein the first opening/closing timing is set such that the period for which the exhaust valve is open overlaps the period for which an intake valve associated with the same cylinder that the exhaust valve is associated with is open, and that opening start timing for the exhaust valve is after bottom dead center of expansion stroke of a piston in the cylinder, and the second opening/closing timing is set such that opening start timing for the exhaust valve is advanced compared with the opening start timing according to the first opening/closing timing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:

FIG. 1 is an overall structural view of an engine system to which a control device for a diesel engine according to one embodiment of the present invention is applied;

FIG. 2 is a top view showing a first rocker arm and a second rocker arm, which are components of a valve timing switching mechanism, before assembled,

FIG. 3 is a top view of the valve timing switching mechanism,

FIG. 4 is a side view showing the second rocker arm after assembled,

FIG. 5 is a partially sectional view showing the valve timing switching mechanism with an operating piston not activated,

FIG. 6 is a partially sectional view showing the valve timing switching mechanism with the operating piston activated,

FIG. 7 is a graph showing a relation between valve lift and opening/closing timing for an exhaust valve, when driven by a first cam and when driven by a second cam,

FIG. 8 is a graph showing how air excess ratio and exhaust temperature vary when opening/closing timing for an exhaust valve is gradually retarded,

FIG. 9 is a graph showing how air excess ratio and exhaust temperature vary when opening/closing timing for an exhaust valve is gradually retarded in a comparative example belonging to prior art, and

FIG. 10 is a flowchart of exhaust temperature raise control performed by an ECU.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings attached, one embodiment of the present invention will be described.

A control device for a diesel engine according to the embodiment of the present invention is installed on a vehicle. FIG. 1 is an overall structural view of an engine system to which the control device is applied. First, referring to FIG. 1, the configuration of the engine system will be described.

A diesel engine (hereinafter referred to as an engine) 1 has a high-pressure accumulator (hereinafter referred to as a common rail) 2 that is provided commonly to cylinders. High-pressure fuel supplied by a fuel injection pump, not shown, and stored in the common rail 2 is supplied to fuel injectors 4 provided for the respective cylinders, and injected into the cylinders from the fuel injectors 4.

A turbocharger 8 is incorporated in an intake passage 6, and intake air sucked in through an air cleaner, not shown, flows into a compressor 8 a of the turbocharger 8 from the intake passage 6. The intake air turbo-charged by the compressor 8 a is passed through an intercooler 10 and introduced to an intake manifold 12. The air introduced to the intake manifold 12 is sucked into the cylinders of the engine 1 through intake ports (not shown) by opening intake valves (not shown), provided for the respective cylinders. An intake air flow sensor 14 for detecting a flow rate of the intake air sucked into the engine 1 is interposed in the intake passage 6, upstream of the compressor 8 a.

Exhaust ports (not shown), through which exhaust gas is discharged from the respective cylinders by opening exhaust valves (not shown in FIG. 1), are connected to an exhaust pipe 18 through an exhaust manifold 16. Disposed between the exhaust manifold 16 and the intake manifold 12 is an EGR passage 22 that connects the exhaust manifold 16 and the intake manifold 12 to each other with an EGR valve 20 interposed therebetween.

The exhaust pipe 18 has a turbine 8 b of the turbocharger 8 incorporated therein, and is connected to an exhaust after-treatment device 24. A rotary shaft of the turbine 8 a is mechanically coupled to a rotary shaft of the compressor 8 a, and the turbine 8 b drives the compressor 8 a by receiving exhaust gas passing through the exhaust pipe 18.

The exhaust after-treatment device 24 has an upstream casing 26 and a downstream casing 30, which is located downstream of the upstream casing 26 and connected thereto by a communication passage 28. The upstream casing 26 accommodates a pre-stage oxidizing catalyst 32, and downstream of the pre-stage oxidizing catalyst 32, a particulate filter (hereinafter referred to as a filter) 34 is placed. The filter 34 traps particulates in exhaust gas, thereby purifying the exhaust gas of the engine 1.

The pre-stage oxidizing catalyst 32 oxidizes NO (nitrogen monoxide) in exhaust gas to produce NO₂ (nitrogen dioxide). Since the pre-stage oxidizing catalyst 32 is placed upstream of the filter 34, NO₂ produced in the pre-stage oxidizing catalyst 32 flows into the filter 34. The particulates trapped and accumulated in the filter 34 are oxidized in reaction with the NO₂ supplied from the pre-stage oxidizing catalyst 32. Continuous regeneration of the filter 34 is carried out in this manner.

The downstream casing 30 accommodates an ammonia selective reduction-type NOx catalyst (hereinafter referred to as an SCR catalyst) 36, which adsorbs ammonia in exhaust gas and selectively reduce the NOx (nitrogen oxides) contained in exhaust gas by using the ammonia as a reducing agent, thereby purifying the exhaust gas. Downstream of the SCR catalyst 36, a post-stage catalytic oxidizer 38 is placed within the downstream casing to remove ammonia from the exhaust gas that has flown out of the SCR catalyst 36. The post-stage oxidizing catalyst 38 has also a function of oxidizing CO (carbon monoxide), which is produced when the particulates are burnt in forced regeneration of the filter 34 that will be described later, and then discharging the resultant CO₂ (carbon dioxide) into the atmosphere.

Further, a urea-water injector 40 for injecting urea-water into the exhaust gas in the communication passage 28 is provided on the communication passage 28. The urea-water is supplied to the urea-water injector 40 from a urea-water tank 42 in which the urea-water is stored, by means of a supply pump not shown. The urea-water supplied is injected into the exhaust gas in the communication passage 28 through the urea-water injector 40 in response to the opening and closing actions of the urea-water injector 40.

The atomized urea-water that has been injected from the urea-water injector 40 is hydrolyzed by heat from the exhaust gas, which produce ammonia. The produced ammonia is supplied to the SCR catalyst 36 together with the exhaust gas. The SCR catalyst 36 adsorbs the supplied ammonia, and promotes denitrifying reaction between the ammonia and the NOx contained in the exhaust gas. Consequently, the NOx contained in exhaust gas is reduced, and converted into harmless N₂, etc. If the ammonia does not react with the NOx and flows out of the SCR catalyst 36, the ammonia is removed from the exhaust gas by the post-stage oxidizing catalyst 38.

Within the downstream casing 30, upstream of the SCR catalyst 36, an inlet temperature sensor 44 is provided to detect exhaust temperature near the inlet of the SCR catalyst 36 is provided.

For each cylinder, the engine 1 has a valve timing switching mechanism (not shown in FIG. 1) to switch the opening/closing timing for an exhaust valve provided to each cylinder of the engine 1. The valve timing switching mechanism, which will be described later in detail, is arranged to switch the opening/closing timing for the exhaust valve by supply of a working oil thereto being controlled. A working oil control valve 46 shown in FIG. 1 is provided to control the supply of the working oil to the valve timing switching mechanism.

The engine system configured as described above has an ECU (control means) 48 to perform comprehensive control, including control of operation of the engine 1. The ECU 48 includes a CPU, memory devices, timer-counters, etc. The ECU 48 calculates various control variables, and controls various devices connected to the ECU 48 according to the calculated control variables.

To the input of the ECU 48, there are connected various sensors, including a water temperature sensor 50 for detecting the temperature of cooling water of the engine 1, a revolution speed sensor 2 for detecting the revolution speed of the engine 1, an accelerator opening sensor 54 for detecting accelerator depression amount, in addition to the above-mentioned intake flow rate sensor 14 and inlet temperature sensor 44, to collect information required for various controls.

To the output of the ECU 48, there are connected various devices controlled according to the calculated control variables, such as the fuel injectors 4 for the respective cylinders, the EGR valve 20, the urea-water injector 40 and the working oil control valve 46.

The ECU 48 also performs fuel supply control, in which the ECU 48 calculates fuel supply quantity for each cylinder of the engine 1 and controls each fuel injector 4 according to the calculated fuel supply quantity. Fuel supply quantity (main injection quantity) required for operation of the engine 1 is determined from a prestored map on the basis of the revolution speed of the engine 1 detected by the revolution speed sensor 52 and the accelerator opening detected by the accelerator opening sensor 54. The amount of fuel supplied to each cylinder is regulated according to the period of time for which the fuel injector 4 is open. The ECU 48 opens the respective fuel injectors 4 for the period of time corresponding to the determined fuel amount, to thereby perform the main fuel injection into the corresponding cylinders. As a result of such main fuel injection, the fuel in the amount required for operation of the engine 1 is supplied.

In addition to the above-described fuel supply control, the ECU 48 performs forced regeneration control to forcedly regenerate the filter 34 to restore its function.

Particulates accumulated in the filter 34 is oxidized to be removed by the above-mentioned continuous regeneration using NO₂ supplied from the pre-stage oxidizing catalyst 32. However, occasionally, the particulates accumulated in the filter 34 cannot be removed sufficiently only by such continuous regeneration. Prolonging this condition leads to excessive accumulation of particulates in the filter 34, which may lead to blocking of the filter 34. Thus, the ECU 48 performs forced regeneration by causing an appropriate rise in the temperature of the filter 34, depending on the level of accumulation of the particulates in the filter 34, thereby maintaining the exhaust purifying function of the filter 34.

The level of accumulation of the particulates is estimated from the pressure difference across the filter 34, the value detected by the intake air flow sensor 14 or the like. When determining that the accumulation of the particulates in the filter 34 has reached a predetermined amount, the ECU 48 starts forced regeneration control. In the forced regeneration control, the ECU 48 performs post injection by controlling the fuel injectors 4, thereby supplying HC (hydrocarbon) into the exhaust gas. Oxidation reaction of the HC thus supplied at the pre-stage oxidizing catalyst 32 causes a rise in temperature of the exhaust gas flowing into the filter 34. Such rise in exhaust temperature allows burning-off of the particulates accumulated in the filter 34.

The ECU 48 also performs urea-water supply control to maintain the exhaust purifying performance of the SCR catalyst 36 at a high level by properly regulating the amount of urea-water supplied from the urea-water injector 40. In the urea-water supply control, the ECU 48 determines, as target supply quantity for urea-water, the amount required for selectively reducing NOx in exhaust gas, and controls the urea-water injector 40 according to this target supply quantity. By this control, urea-water is supplied from the urea-water injector 40 into the exhaust gas existing upstream of the SCR catalyst 36.

As mentioned above, urea-water that has been supplied from the urea-water injector 40 is hydrolyzed by heat from the exhaust gas, and the resultant ammonia is supplied to the SCR catalyst 36. The SCR catalyst 36 adsorbs the supplied ammonia, and promotes denitrifying reaction between the ammonia and the NOx contained in the exhaust gas, so that the NOx is reduced and therefore the exhaust gas is purified.

In order for the above-described forced regeneration of the filter 34 and selective reduction of NOx at the SCR catalyst 36 to be performed satisfactorily, the pre-stage oxidizing catalyst 32 and the SCR catalyst 36 are required to be activated. Further, for the selective reduction of NOx at the SCR catalyst 36, it is required that the urea-water supplied from the urea-water injector 40 into the exhaust gas be hydrolyzed by heat from the exhaust gas, to a sufficient extent, in order to supply an appropriate amount of ammonia to the SCR catalyst 36. Thus, the temperature of the exhaust gas of the engine 1 needs to have risen to a level allowing the pre-stage oxidizing catalyst 32 and the SCR catalyst 36 to be activated.

Thus, if the engine 1 is operating in a condition not allowing the exhaust temperature to reach such level, the control device according to the present embodiment causes a rise in exhaust temperature by changing the opening and closing timing for the exhaust valves.

Next, on the basis of FIGS. 2 to 7, the configuration of the valve timing switching mechanism 56 provided for each cylinder to change the opening/closing timing for the exhaust valve will be described in detail.

FIG. 2 is a top view showing a first rocker arm 58 and a second rocker arm 60, which are components of the valve timing switching mechanism 56, before assembled. FIG. 3 is a top view of the valve timing switching mechanism 56, and FIG. 4 is a side view showing the second rocker arm after assembled.

For each cylinder of the engine 1, there are provided a first rocker arm 18 and a second rocker arm 60, which are swingably supported by a rocker shaft 62, adjacent to each other, individually.

The first rocker arm 58 has a boss portion 64 allowing the rocker shaft 62 to pass through, so that the first rocker arm 58 is supported by the rocker shaft 62 at the boss portion 64. Further, the first rocker arm 58 has a shaft portion 66 projecting from the boss portion 64 along the axis of the rocker shaft 62, which allows the rocker shaft 62 to pass through and which is inserted in a boss portion 68 of the second rocker arm 60. Thus, the second rocker arm 60 is supported by the shaft portion 66 of the first rocker arm 58 in a manner such that the second rocker arm 60 is swingable relative to the rocker shaft 62.

A roller 74 that contacts a first cam 72 is rotatably fitted to an end portion of an arm 70 that extends along the length of the first rocker arm 58 to one side. To an end portion of an arm 76 extending to the opposite side to the arm 70, a valve stem of the exhaust valve 78 is connected. The roller 74 of the first rocker arm 58 is pressed to the first cam 72 by a force exerted by a valve spring (not shown) fitted to the exhaust valve 78 connected to the arm 76 of the first rocker arm 58.

A roller 84 that contacts a second cam 82 is rotatably fitted to an end portion of an arm 80 that extends from the boss portion 68 of the second rocker arm 60 in the same direction as the arm 70 of the first rocker arm 58 extends. The second cam 82 is different in cam profile from the first cam 72. The second rocker arm 60 is forced by a return spring 88 fitted on an end of a thickened portion 86 formed on the boss portion 68, so that the roller 84 is pressed to the second cam 82.

In the boss portion 64 of the first rocker arm 58, a cylinder portion 90 having an axis approximately at right angles to the axis of the rocker shaft 62 is provided. Within the cylinder portion 90, an operating piston 92 is slidably fitted. As described later, the operating piston 92 is activated by the pressure of a working oil supplied to an oil chamber 96 defined under the operating piston 92, through an oil passage 94 provided in the rocker shaft 62. When the operating piston 92 is not activated, namely when the working oil is not supplied to the oil chamber 96, the operating piston 92 is located low within the cylinder portion 90 by being pressed by a return spring 98 as shown in FIG. 5. When the working oil is supplied to the oil chamber 96, the pressure of the working oil moves the operating piston 92 upward within the cylinder portion 90, against the return spring 98, as shown in FIG. 6.

As seen in FIGS. 5 and 6, the operating piston 92 has an engaging groove 104 consisting of a deep groove portion 100 and a shallow groove portion 102. In a manner corresponding to this engaging groove 104, an engaging projection 106 projects from the second rocker arm 60 toward the first rocker arm 58 to extend over the arm 70 of the first rocker arm 58 and further toward the operating piston 92.

As shown in FIG. 6, when the working oil is supplied to the oil chamber 96 so that the operating piston 92 is held high within the cylinder portion 90, the swing of the second rocker arm 60 driven by the second cam 82 causes the engaging projection 106 to advance into the shallow groove portion 102 and contact the operating piston 92 in the direction of the swing. By the engaging projection 106 contacting the operating piston 92 in this manner, the swing of the second rocker arm 60 is transmitted to the first rocker arm 58.

In contrast, as shown in FIG. 5, when the working oil is not supplied to the oil chamber 96 so that the operating piston 92 is located low within the cylinder portion 90, the swing of the second rocker arm 60 driven by the second cam 82 causes the engaging projection 106 to advance into the deep groove portion 100 of the operating piston 92. The engaging projection 106 does not contact the operating piston 92 in this case, so that the swing of the second rocker arm 60 is not transmitted to the first rocker arm 58.

As mentioned above, the working oil is supplied to the oil chamber 96 and discharged from the oil chamber 96 by the ECU 48 controlling the working oil control valve 46.

The cam profiles of the first and second cams 72 and 82 are determined such that the amount of the lift of the exhaust valve 78 caused by the first cam 72 is not greater than the amount of the lift of the exhaust valve 78 caused by the second cam 82 at any point of time. Consequently, when the supply of the working oil to the oil chamber 96 causes the engaging projection 106 to contact the operating piston 92 located high within the cylinder portion 90 so that the swing of the second rocker arm 60 is transmitted to the first rocker arm 58, the exhaust valve 78 is opened and closed by the first rocker arm 58 swinging according to the cam profile of the second cam 82.

In contrast, when the working oil is not supplied to the oil chamber 96 so that the operating piston 92 is located low within the cylinder portion 90, the exhaust valve 78 is opened and closed by the first rocker arm 58 swinging according to the cam profile of the first cam 72, since the swing of the second rocker arm 60 is not transmitted to the first rocker arm 58 as mentioned above.

In the present embodiment, the cam profiles of the first and second cams 72 and 82 are determined such that the exhaust valve 78 is driven by either one of the cams 72 and 82 according to characteristics of a lift amount and opening/closing timing relative to crank angle as shown in FIG. 7.

Specifically, in FIG. 7, curve EX1 shows the lift amount and the opening/closing timing (first opening/closing timing) for the exhaust valve 78, determined by the cam profile of the first cam 72, while curve EX2 shows the lift amount and the opening/closing timing (second opening/closing timing) for the exhaust valve 78, determined by the cam profile of the second cam 82. Curve IN in FIG. 7 shows the lift amount and the opening/closing timing for the intake valve used in combination with the exhaust valve 78.

As curve EX2 in FIG. 7 shows, the exhaust valve 78 opened and closed by the second cam 82 starts opening before bottom dead center (BDC) of expansion stroke, and completes closing after the intake valve starts opening. Further, the period for which the exhaust valve 78 is open overlaps the period for which the intake valve is open. Such characteristics of the second cam 82 are similar to the characteristics of the exhaust-valve driving cam, used in common diesel engines not having a valve timing switching mechanism.

In contrast, as curve EX1 in FIG. 7 shows, the exhaust valve 78 opened and closed by the first cam 82 starts opening a crank angle A (°) after bottom dead center (BDC) of expansion stroke. The retard quantity A relative to BDC is set to a specific crank angle between 400 and 700, for reasons set forth later. Further, the period for which the exhaust valve 78 is open overlaps the period for which the intake valve is open, and the timing of completion of closing of the exhaust valve 78 is approximately in agreement with the timing of completion of closing of the exhaust valve 78 when driven by the second cam 82.

As clear from the above, opening start timing for the exhaust valve 78 is greatly retarded compared with when the exhaust valve 78 is opened and closed by the second cam 82. However, the maximum lift amount L1 for the exhaust valve 78 opened and closed by the first cam 72 is set to be between ⅕ and ⅓ of the maximum lift amount L2 for the exhaust valve 78 opened and closed by the second cam 82. Consequently, the rate of change of the lift amount for the exhaust valve 78 opened and closed by the first cam 72 is within a range of appropriate values, thereby ensuring that the exhaust valve 78 is opened and closed without difficulties.

Specifically, if the exhaust valve 78, for which only the opening start timing is retarded to be between 40° and 70° after BDC while the closing completion timing remains approximately in agreement with that for the exhaust valve 78 opened and closed by the second cam 82, has the maximum lift amount remaining at L2, the valve spring of the exhaust valve 78 has a greatly reduced load margin, which leads to defective operation of the exhaust valve 78, such as jumping of the exhaust valve 78. In order to prevent such defective operation, the maximum lift amount L1 for the exhaust valve 78 needs to be set between ⅕ and ⅓ of the maximum lift amount L2 for the exhaust valve 78 opened and closed by the second cam 82.

Next, how a rise in exhaust temperature is caused by using the valve timing switching mechanism 56 having the above-described configuration will be described.

FIG. 8 is a graph showing how air excess ratio λ and exhaust temperature Tti are related to opening start timing for the exhaust valve 78, which is obtained by varying the opening/closing timing for the exhaust valve 78 opened and closed by the first cam 72 by varying the cam profile of the first cam 72, or more specifically, by gradually retarding only the opening start timing, compared with the normal opening/closing timing as when the exhaust valve 78 is opened and closed by the second cam 82, under the condition that the engine operates in a medium-speed medium-load condition. In FIG. 8, variation of air excess ratio λ is plotted in solid line, while variation of exhaust temperature Tti is plotted in chain line. The opening start timing for the exhaust valve 78 is specified as a crank angle, where the opening start timing is expressed as 0°, positive degrees and negative degrees if the exhaust valve starts opening when the piston is at BDC, before BDC and after BDC, respectively. The exhaust temperature Tti is the temperature of exhaust gas near the inlet of the turbine 8 b of the turbocharger 8.

FIG. 8 shows that greater retarding of the opening start timing for the exhaust valve 78 results in higher exhaust temperature Tti. The opening start timing of 40° after BDC, namely −40° BBDC (before bottom dead center) results in the exhaust temperature of no less than 400° C and the air excess ratio λ of approximately 2.3, which means that the air excess ratio λ of no less than 1.5 is ensured. The reason why the retarding of the opening start timing for the exhaust valve 78 results in higher exhaust temperature is as follows:

Retarding of the opening start timing for the exhaust valve 78 results in an increase in pumping loss in each cylinder, since the piston compresses gases within the cylinder. In this case, in order to obtain engine output corresponding to the accelerator opening detected by the accelerator opening sensor 54, the ECU 48 increases the amount of fuel supplied into the cylinders from the injectors 4 so that the increase in pumping loss is compensated. This results in an increase in temperature of the exhaust gas discharged from the engine 1.

Further, the opening start timing for the exhaust valve 78 retarded compared with the exhaust valve 78 is opened and closed by the second cam 82 results in an increased amount of gases remaining in the cylinder, which in turn results in a corresponding decrease in the amount of fresh air sucked into the cylinder. Such decrease in the fresh air, to which heat produced by combustion of fuel is transferred, results in an increase in exhaust temperature.

The air excess ratio λ decreases as the opening start timing for the exhaust valve 78 is retarded. The air excess ratio λ is approximately 2.0 even at the opening start timing of 70° after BDC (−70° BBDC), which means that the air excess ratio λ of no less than 1.5 is ensured. The reason why the retarding of the opening start timing for the exhaust valve 78 does not lead to a great decrease in the air excess ratio λ is as follows:

Also when the opening start timing for the exhaust valve 78 is retarded, the period for which the exhaust valve 78 is open and the period for which the intake valve is open overlap as mentioned above, or in other words, the exhaust valve 78 is open after the intake valve is opened to some extent. Thus, although the retarding of the opening start timing for the exhaust valve 78 results in an increase in the amount of gases remaining in the cylinder, the increase is not great. Accordingly, the remaining gases do not notably hinder the supply of fresh air into the cylinder, and the supply of fresh air not experiencing a great decrease ensures a sufficient air excess ratio.

It is known that the air excess ratio λ decreased to less than 1.5 is liable to produce black smoke. However, as mentioned above, at the opening start timing of 70° after BDC, the air excess ratio λ is approximately 2.0, therefore, the air excess ratio λ of no less than 1.5 is ensured. Thus, although retarding of the opening start timing for the exhaust valve 78 entails a decrease in air excess ratio, the decrease in air excess ratio does not lead to production of black smoke. Further, retarding of the opening start timing to 70° after BDC brings about a further rise in the exhaust temperature, so that the exhaust temperature reaches 500° C. or above. Thus, setting of the opening start timing for the exhaust valve 78 to a specific crank angel between 40° and 70° after BDC can bring about a desirable rise in the exhaust temperature, while maintaining the air excess ratio λ at a level not leading to production of black smoke.

It is to be noted that the exhaust gas warms up to higher temperature when the opening start timing for the exhaust valve 78 is retarded in this manner, compared with when the opening start timing is not retarded. Thus, in the present embodiment, out of consideration for reliability of the engine 1, the ECU 48 has a function of stopping fuel injection from the injectors 4 when the exhaust temperature exceeds a predetermined allowable upper limit, thereby preventing excessive rise in the exhaust temperature.

For comparison with the present embodiment, FIG. 9 shows a graph which shows how the air excess ratio λ and the exhaust temperature Tti are related to the opening start timing for an exhaust valve, in a similar manner to the graph of FIG. 8, but which is obtained by varying the opening/closing timing for the exhaust valve by varying the phase of the exhaust cam shaft relative to the crank shaft, or more specifically, by advancing the opening start timing for the exhaust valve while fixing valve opening period, as in the device disclosed in Document 1 mentioned above.

FIG. 9 shows that greater advancing of both opening start timing and closing completion timing results in higher exhaust temperature Tti, however entails great decrease of air excess ratio λ. The reason for such great decrease of the air excess ratio λ is as follows:

In this case, advancing of the opening start timing for the exhaust valve entails advancing of the closing completion timing, so that the exhaust valve closes before the intake valve is opened. This results in a large amount of gases remaining in the cylinder, which notably hinders fresh air from being sucked into the cylinder, thereby causing a great decrease in the air excess ratio λ.

The decrease in the air excess ratio λ is so great that, for example, an attempt to cause a rise in the exhaust temperature to 600° C. entails a decrease in the air excess ratio λ to approximately between 1.3 and 1.4, and therefore production of black smoke.

In contrast, the present embodiment can cause a rise in the exhaust temperature in a good manner, without decreasing the air excess ratio λ to such an extent that leads to producing of black smoke. This demonstrates significant superiority of the control device according to the present embodiment over the comparative example having the characteristics shown in FIG. 9.

The ECU 48 performs exhaust temperature raise control using the above-described valve timing switching mechanism 56, depending on the operating condition of the engine 1. This exhaust temperature raise control is performed according to the flowchart of FIG. 10 in control cycles of a predetermined period while the engine 1 is operating.

At the start of the exhaust temperature raise control, the ECU 48 first determines in Step S1 whether or not the engine 1 is operating in a cold condition on the basis of the value detected by the water temperature sensor 50. If the temperature of the cooling water of the engine 1, detected by the water temperature sensor 50 is below a predetermined reference water temperature, the ECU 48 determines that the engine 1 is operating in a cold condition. When the working oil supplied through the working oil control valve 46 to the valve timing switching mechanism 56 is at low temperature, there is a possibility that the valve timing switching mechanism 56 does not function properly because of high viscosity of the working oil. Thus, the reference water temperature is predetermined on the basis of a reference oil temperature set to the lower limit that allows the valve timing switching mechanism 56 to function properly. Thus, by determining that the value detecting by the water temperature sensor 50 is below the reference water temperature, the ECU 48 determines that the engine 1 is operating in a cold condition with the working oil below the reference oil temperature, and advances the procedure to Step S2.

In Step S2, in order to select the first cam 72 to drive the exhaust valve 78, the ECU 48 controls the working oil control valve 46 not to supply the working oil to the valve timing switching mechanism 56, and finishes the current control cycle. In the valve timing switching mechanism 56, since the working oil is not supplied, the operating piston 92 of the first rocker arm 58 is located low within the cylinder portion 90 as mentioned above. The engaging projection 106 of the second rocker arm 60 therefore advances into the deep groove portion 100 of the operating piston 92 and does not contact the operating piston 92. Thus, the swing of the second rocker arm 60 is not transmitted to the first rocker arm 58, and the first rocker arm 58 is driven by the first cam 72 so that the exhaust valve 78 is opened and closed according to the cam profile of the first cam 72. Consequently, the opening start timing for the exhaust valve 78 is retarded to a specified crank angle between 40° and 70° after BDC as mentioned above, which causes a rise in exhaust temperature.

In the subsequent control cycles, as long as determining in Step S1 that the engine 1 is operating in a cold condition, the ECU advances the procedure to Step S2. Thus, the exhaust valve 78 is opened and closed by the first cam 72 in the above-described manner, so that rise in exhaust temperature is caused continuously.

Generally, when the engine 1 is operating in a cold condition, the exhaust after-treatment device 24 is also at low temperature. Causing a rise in exhaust temperature in the above-described manner when the engine 1 is operating in a cold condition allows the pre-stage oxidizing catalyst 32, the SCR catalyst 36 and the post-stage oxidizing catalyst 38 to be activated early. Further, this also causes a rapid rise in temperature of the filter 34, therefore allows continuous generation of the filter 34 to start early. Further, as mentioned above, production of black smoke is well suppressed while the exhaust temperature rise is caused in this manner.

Further, when the ECU 48 determines that the engine 1 is operating in a cold condition, namely that the working oil has not reached the reference oil temperature, the exhaust valve 78 is opened and closed by the first cam 72, without supply of the working oil. Thus, the exhaust temperature rise can be caused, without the operation of the valve timing switching mechanism 56 becoming unstable due to the working oil at low temperature.

Warm-up operation of the engine 1 causes a rise in cooling water temperature of the engine 1. If the ECU 48 determines in Step S1 that the value detected by the water temperature sensor 50 is no less than the reference water temperature, namely the engine is not operating in a cold condition, this means that it is determined that the working oil has reached the reference oil temperature that allows stable operation of the valve timing switching mechanism 56. In this case, the ECU 48 advances the procedure to Step S3.

In Step S3, the ECU 48 determines whether or not the exhaust temperature Tex detected by the inlet temperature sensor 44 is no less than a predetermined reference exhaust temperature Ts. This reference exhaust temperature Ts is predetermined on the basis of the temperature that allows not only the SCR catalyst 36 but also the pre-stage oxidizing catalyst 48 and the post-stage oxidizing catalyst 38 to be activated. If the ECU 48 determines that the exhaust temperature Tex is no less than the reference exhaust temperature Ts, the exhaust temperature has reached the level that allows the SCR catalyst 36, the pre-stage oxidizing catalyst 48 and the post-stage oxidizing catalyst 38 to be activated.

Thus, if, in Step S3, the ECU 48 determines that the exhaust temperature Tex is less than the reference exhaust temperature Ts, the exhaust temperature has not reached the level that allows the above catalysts to be activated. Thus, the ECU 48 advances the procedure to Step S2, selects to open and close the exhaust valve 78 by the first cam 72, and finishes the current control cycle. Consequently, exhaust temperature rise is caused in the above-described manner.

Also in the subsequent control cycles, as long as determining in Step S1 that the engine 1 is not operating in a cold condition and then determining in Step S3 that the exhaust temperature Tex has not reached the reference exhaust temperature Ts, the ECU 48 advances the procedure to Step S2. Thus, the exhaust valve 78 is opened and closed by the first cam 72 in the above-described manner, so that rise in exhaust temperature is caused continuously.

Thus, if the exhaust temperature has not reached the level that allows the above-mentioned catalysts to be activated although the engine 1 is no longer operating in a cold condition, or if a change in engine operating conditions causes a decrease in exhaust temperature to below the level allowing the above-mentioned catalysts to be activated, rise in exhaust temperature is caused by opening and closing the exhaust valve 78 by the first cam 72. This enables rapid rise in temperature of the above-mentioned catalysts to the level allowing their activation.

If determining in Step S3 that the exhaust temperature Tex is no less than the reference exhaust temperature Ts, the ECU 48 advances the procedure to Step S4. In Step S4, the ECU 48 determines whether or not the vehicle is decelerating on the basis of the engine revolution speed detected by the revolution speed sensor 52 and the accelerator opening detected by the accelerator opening sensor 54.

In the deceleration of the vehicle, the engine 1 is also decelerating, so that the engine 1 comes to operate in a low-load condition, or fuel injection from the injectors 4 is stopped. This causes a decrease in temperature of the exhaust gas discharged from the engine 1, so that the exhaust gas at decreased temperature is supplied to the exhaust after-treatment device 24.

Thus, if the ECU 48 determines in Step S4 that the vehicle is decelerating, the engine 1 is operating in a condition that causes a decrease in exhaust temperature. In this case, the ECU advances the procedure to Step S2. I Step S2, the ECU 48 selects to open and close the exhaust valve 78 by the first cam 72 as mentioned above, and finishes the current control cycle.

Also in the subsequent control cycles, as long as determining in Step S1 that the engine 1 is not operating in a cold condition, then determining in Step S3 that the exhaust temperature Tex has reached the reference exhaust temperature Ts, and then determining in Step S4 that the vehicle is decelerating, the ECU 48 advances the procedure to Step S2. Consequently, the exhaust valve 78 is opened and closed by the first cam 72 in the above-described manner.

As mentioned above, if the ECU 48 selects to open and close the exhaust valve 78 by the first cam 72, the opening start timing for the exhaust valve 78 is retarded. This results in an increase in the amount of gases remaining in the cylinder, compared with when the exhaust valve 78 is opened and closed by the second cam 82, and a corresponding decrease in the amount of fresh air sucked into the cylinder. This in turn results in a decrease in the amount of exhaust gas supplied to the exhaust after-treatment device 24, which can suppress the decrease in temperature of the after-treatment device 24.

Further, if the ECU 48 selects to open and close the exhaust valve 78 by the first cam 72 when the vehicle is decelerating, the opening start timing for the exhaust valve 78 is retarded, which also results in an increase in pumping loss, since the piston compresses gases in the cylinder. In the vehicle deceleration, such increase in pumping loss provides an engine braking effect.

If determining in Step S4 that the vehicle is not decelerating, the ECU 48 advances the procedure to Step S5. In Step S5, the ECU 48 selects to open and close the exhaust valve 78 by the second cam 82, and finishes the current control cycle. Thus, the above-described process of causing a rise in exhaust temperature by retarding the opening start timing for the exhaust valve 78 is not carried out, and the engine 1 is operated in a normal manner.

Under the exhaust temperature raise control performed by the ECU 48 in the above-described manner, when the engine 1 is operating in a cold condition or when the exhaust temperature Tex has not reached the reference exhaust temperature Ts, a rapid rise in exhaust temperature can be achieved by opening and closing the exhaust valve 78 by the first cam 72, thereby retarding the opening start timing for the exhaust valve 78.

Particularly when the engine 1 is operating in a cold condition, the working oil for operating the valve timing switching mechanism 56 is at low temperature and therefore high in viscosity. Under this condition, switching the working position of the valve timing switching mechanism 56 by supply or discharge of the working oil may hinder stable operation of the valve timing switching mechanism 56. In the present embodiment, however, a switch from the opening and closing of the exhaust valve 78 by the first cam 72 to the opening and closing of the exhaust valve 78 by the second cam 82 is prohibited until the cooling water temperature detected by the water temperature sensor 50 has risen to the reference water temperature or above, namely the working oil has warmed to the reference oil temperature or above. It is to be noted that the opening and closing of the exhaust valve 78 by the first cam 72 does not require supply of the working oil. This means that the valve timing switching mechanism 56 can function stably even when the engine is operating in a cold condition.

When the engine 1 is operating in a cold condition so that the exhaust valve 78 is opened and closed by the first cam 72 as mentioned, the period for which the exhaust valve 72 is open overlaps the period for which the intake valve is open, or in other words, the exhaust valve continues to be open after the intake valve is opened to some extent. This prevents the supply of fresh air into the cylinder from being notably hindered by the gases remaining in the cylinder, thereby preventing a great decrease in the amount of fresh air sucked into the cylinder. Thus, if, in this situation, the engine 1 comes to operate under a high load, the exhaust temperature does not rise excessively.

Also when the vehicle is decelerating, the exhaust valve 78 is opened and closed by the first cam 72, namely the opening start timing for the exhaust valve 78 is retarded. This results in a decrease in the amount of exhaust gas supplied to the exhaust after-treatment device 24, which can suppress a decrease in temperature of the exhaust after-treatment device 24 and produce the engine braking effect.

In the above, the control device for a diesel engine according one embodiment of the present invention has been described. The present invention is, however, not restricted to the described embodiment.

For example, although in the described embodiment, the exhaust after-treatment device 24 includes a pre-stage oxidizing catalyst 32, a filter 34, an SCR catalyst 36 and a post-stage oxidizing catalyst 38, the configuration of the exhaust after-treatment device 24 is not restricted to this but can be modified as necessary. In other words, application of the present invention to a diesel engine can produce the similar effects, as long as the diesel engine is equipped with an exhaust after-treatment device which may be unable to perform exhaust purifying function satisfactorily due to a decrease in exhaust temperature.

Further, in the described embodiment, the ECU 48 determines, in Step S4 of the exhaust temperature raise control, that the engine 1 is operating in a condition causing a decrease in exhaust temperature, by determining that the vehicle is decelerating, and selects to open and close the exhaust valve 78 by the first cam 72. The manner of determining whether or not the engine 1 is operating in a condition causing a decrease in exhaust temperature is, however, not restricted to this. For example, it may be arranged such that when the engine 1 is decelerating or operating in a predetermined low-speed low-load condition, the ECU 48 may determine that the engine 1 is operating in a condition causing a decrease in exhaust temperature.

Incidentally, whether or not the engine 1 is decelerating may be determined, for example on the basis of the engine revolution speed detected by the revolution speed sensor 52, as mentioned above, or on the basis of the supply of fuel to the engine 1. Further, the ECU 48 may determine whether or not the engine is operating in a predetermined low-speed low-load condition on the basis of the reference load and revolution speed which result in the exhaust temperature leading to decline of the exhaust purifying function of the exhaust after-treatment device, where such reference load and revolution speed are obtained by way of experiment in advance.

Further, when it is determined whether or not the vehicle is decelerating as in the described embodiment, the way of determination is not restricted to that adopted in the described embodiment. For example, whether or not the vehicle is decelerating may be determined on the basis of the rate of change of traveling speed of the vehicle, or on the basis of the manipulation for deceleration which is performed by the driver of the vehicle.

Further, in the described embodiment, the valve timing switching mechanism 56 includes a first rocker arm 58 driven by a first cam 72 and a second rocker arm 60 driven by a second cam 82, and the valve timing switching mechanism 56 is configured to change the opening/closing timing for the exhaust valve 78 by allowing or blocking transmission of the swing of the second rocker arm 60 to the first rocker arm 58. The configuration of the valve timing switching mechanism 56 is, however, not restricted to this. In other word, any configuration may be adopted as long as it allows a switch between first opening/closing timing and second opening/closing timing in the present invention.

Further, in the described embodiment, the ECU 48 determines that the engine 1 is operating in a cold condition on the basis of cooling water temperature of the engine 1 detected by the water temperature sensor 50. The way to determine that the engine is operating in a cold condition is not restricted to this. For example, whether or not the engine 1 is operating in a cold condition may be determined on the basis of the temperature of the working oil used for operating the valve timing switching mechanism 56 or the temperature of the cylinder block of the engine 1.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A control device for a diesel engine, comprising: a valve timing switching mechanism capable of selectively switching between first opening/closing timing and second opening/closing timing for an exhaust valve of the diesel engine; an exhaust after-treatment device for purifying exhaust gas discharged from the diesel engine; and a control means for controlling the valve timing switch mechanism so that the exhaust valve is opened and closed according to the first opening/closing timing, at least when the diesel engine is operating in a predetermined cold condition, wherein the first opening/closing timing is set such that the period for which the exhaust valve is open overlaps the period for which an intake valve associated with the same cylinder that the exhaust valve is associated with is open, and that opening start timing for the exhaust valve is after bottom dead center of expansion stroke of a piston in the cylinder, and the second opening/closing timing is set such that opening start timing for the exhaust valve is advanced compared with the opening start timing according to the first opening/closing timing.
 2. The control device for a diesel engine according to claim 1, wherein the opening start timing for the exhaust valve according to the first opening/closing timing is set to a predetermined crank angle of the piston between 40° and 70° after bottom dead center.
 3. The control device for a diesel engine according to claim 1, wherein the exhaust valve is opened and closed according to the second opening/closing timing when a working oil is supplied to the valve timing switching mechanism, and the exhaust valve is opened and closed according to the first opening/closing timing when the working oil is not supplied to the valve timing switching mechanism.
 4. The control device for a diesel engine according to claim 3, wherein when the control means is controlling the valve timing switching mechanism so that the exhaust valve is opened and closed according to the first opening/closing timing, the control means prohibits the valve timing switching mechanism from switching from the first opening/closing timing to the second opening/closing timing until determining that the temperature of the working oil has risen to a predetermined oil temperature or above.
 5. The control device for a diesel engine according to claim 1, wherein when the control means determines that the diesel engine is operating in a predetermined condition that causes a decrease in the temperature of the exhaust gas, the control means controls the valve timing switching mechanism so that the exhaust valve is opened and closed according to the first opening/closing timing.
 6. The control device for a diesel engine according to claim 5, wherein the diesel engine is mounted on a vehicle as a power source; and when the vehicle is decelerating, the control means determines that the diesel engine is operating in the predetermined condition that causes a decrease in the temperature of the exhaust gas.
 7. The control device for a diesel engine according to claim 5, wherein when the diesel engine is decelerating, the control means determines that the diesel engine is operating in the predetermined condition that causes a decrease in the temperature of the exhaust gas.
 8. The control device for a diesel engine according to claim 5, wherein when the diesel engine is operating in a predetermined low-speed low-load condition, the control means determines that the diesel engine is operating in the predetermined condition that causes a decrease in the temperature of the exhaust gas.
 9. The control device for a diesel engine according to claim 5, wherein when the temperature of the exhaust gas of the diesel engine is below a predetermined exhaust temperature, the control means determines that the diesel engine is operating in the predetermined condition that causes a decrease in the temperature of the exhaust gas. 